Numerous publications--for example, "Adaptive Cruise Control System Aspects and Development Trends" by Winner et al., SAE 961010, presented at SAE 1996 in Detroit--describe equipping motor vehicles with a clearance sensor. A clearance sensor of this kind can be implemented, for example, as a radar sensor, laser sensor, or infrared sensor, and serves to detect obstacles which are located in the path of the vehicle. Usually, at least one distance between the obstacle and the motor vehicle is also determined. Such sensors are used as simple obstacle or clearance warning devices, and also in the context of intelligent or adaptive cruise control (AICC or ACC) systems in which the speed of a regulated vehicle is automatically adapted to the speed of any vehicles traveling ahead of it. A further application for such sensors is the monitoring or observation of a region of space as a lane-changing aid.
All these applications have in common the fact that the clearance sensor used must be accurately aligned, so that its "direction of view" matches a direction being monitored. This matching requirement applies equally to both horizontal and vertical alignment. If a clearance sensor of this kind is intended to be capable of detecting an azimuthal direction of detected targets in addition to mere detection, the clearance sensor must have a high-resolution radiation characteristic in the azimuthal plane. This means that the horizontal alignment must be performed very precisely and accurately, and yet offer a simple yet precise method for checking the horizontal alignment; for example with reference to a known reference target and utilizing the sensor's own directional analysis system.
In contrast, the requirements for the accuracy of a vertical alignment are generally not so stringent. However, due to the limited transmission power available with such clearance sensors, the vertical radiation characteristic is also made as narrow as possible with reasonable effort. As a result, the vertical alignment must also be very accurate. If the clearance sensor, for example, radiates a transmitted signal at too steep an angle, i.e., too high, targets that might be in the path of the vehicle will not be illuminated by that radiation and accordingly will not be detected. Obstacles may also not be accurately detected if the clearance sensor radiates too low, i.e., toward the ground.
Conventional methods for checking a vertical alignment of a clearance sensor of this kind are based on a stationary measurement of the overall radiation characteristic of the clearance sensor in which a reference target (e.g., a reflector, or a measurement receiver) is positioned at a desired height opposite the clearance sensor. The clearance sensor is then aligned vertically in such a way that it illuminates the reference target or measurement receiver with a maximum transmission power. The principal disadvantage of this method is that a complex measurement structure is needed to merely check the alignment. A horizontal misalignment of the clearance sensor can be recognized, with a high probability, by the erroneous angle of the targets. However, in the case of a vertical misalignment, no targets are detected if the sensor is set too high or too low. Therefore, it is possible that a vertical misadjustment will not recognized in some situations.